1. Field of the Invention
The present invention relates to a high-intensity discharge lamp ballast and a method for operating a high-intensity discharge lamp.
2. Discussion of the Background
A high-intensity discharge lamp is a light source which allows brighter lighting with higher light output and is operated with a device called ballast in order to achieve more steady lighting. There are two types of ballasts: a magnetic ballast composed mainly of inductors, and an electronic ballast having the electronic circuit which provides switching control, and the electronic ballast is more popular these days for the energy-saving objective.
U.S. Pat. No. 6,426,597 discloses an electronic ballast for operating a gas discharge lamp, and this electronic ballast has a circuit arrangement comprised of four switches interconnected to form a full bridge.
Also, U.S. Pat. No. 6,448,720 discloses a discharge lamp driving circuit having a tank circuit and a DC-AC inverter, and the DC-AC inverter has a bridge circuit in which MOSFETs are configured in a full bridge arrangement.
Incidentally, when a driving circuit starts operating a high-intensity discharge lamp, since the temperature and the pressure inside the lamp are still low, the lamp does not generate sufficient light output. When the temperature and the pressure become considerably high, the lamp begins its steady operation, but at this point, if the high-intensity discharge lamp is turned off, it takes time to restart the lamp. For example, it may take several minutes or several tens of minutes until the temperature and the pressure become lower again. Thus, if the high-intensity discharge lamp is switched off due to a blackout, for example, and even if the power gets back immediately after, the lamp may not restart its operation with sufficient light output for several tens of minutes.
U.S. Pat. No. 6,489,729 discloses an auxiliary lighting device which turns on an auxiliary lamp while a high-intensity discharge lamp is turned off. The auxiliary lighting device is provided with an HID lamp status circuit, a phase control circuit, a TRIAC, an auxiliary light source, a rectifier circuit and an amplifier. When the HID lamp status circuit receives a signal indicating that the high-intensity discharge lamp is “OFF”, the HID lamp status circuit sends a signal to the phase control circuit to turn on the auxiliary light source. The rectifier circuit and the amplifier form a feedback circuit, and the rectifier circuit outputs a voltage signal having a magnitude of the voltage applied to the auxiliary light source. The amplifier compares the voltage signal with a reference voltage, and outputs to the phase control circuit an error signal representing the difference between the voltage signal and the reference voltage. Based on the error signal, the phase control circuit controls the TRIAC so that a target voltage is applied to the auxiliary light source. However, this auxiliary lighting device requires the detection of the voltage across the auxiliary light source followed by the feedback control based on the detected voltage, and therefore the performance of the auxiliary lighting device may be less efficient, and its circuit structure tends to be more complicated and costly.
The contents of the U.S. Pat. Nos. 6,426,597, 6,448,720 and 6,489,729 are incorporated herein by reference in their entirety.